A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned.
Lithography is widely recognized as one of the key steps in the manufacture of ICs and other devices and/or structures. However, as the dimensions of features made using lithography become smaller, lithography is becoming a more critical factor for enabling miniature IC or other devices and/or structures to be manufactured.
A theoretical estimate of the limits of pattern printing can be given by the Rayleigh criterion for resolution as shown in equation (1):
                    CD        =                              k            1                    *                      λ            NA                                              (        1        )            where λ is the wavelength of the radiation used, NA is the numerical aperture of the projection system used to print the pattern, k1 is a process dependent adjustment factor, also called the Rayleigh constant, and CD is the feature size (or critical dimension) of the printed feature. It follows from equation (1) that reduction of the minimum printable size of features can be obtained in three ways: by shortening the exposure wavelength λ, by increasing the numerical aperture NA or by decreasing the value of k1.
In order to shorten the exposure wavelength and, thus, reduce the minimum printable size, it has been proposed to use an extreme ultraviolet (EUV) radiation source. EUV radiation is electromagnetic radiation having a wavelength within the range of 5-20 nm, for example within the range of 13-14 nm. It has further been proposed that EUV radiation with a wavelength of less than 10 nm could be used, for example within the range of 5-10 nm such as 6.7 nm or 6.8 nm. Such radiation is termed extreme ultraviolet radiation or soft x-ray radiation. Possible sources include, for example, laser-produced plasma sources, discharge plasma sources, or sources based on synchrotron radiation provided by an electron storage ring.
EUV radiation may be produced using a plasma. A radiation system for producing EUV radiation may include a laser for exciting a fuel to provide the plasma, and a source collector module for containing the plasma. The plasma may be created, for example, by directing a laser beam at a fuel, such as particles (i.e. droplets) of a suitable fuel material (e.g. tin), or a stream of a suitable gas or vapor, such as Xe gas or Li vapor. The resulting plasma emits output radiation, e.g., EUV radiation, which is collected using a radiation collector. The radiation collector may be a mirrored normal incidence radiation collector, which receives the radiation and focuses the radiation into a beam. The source collector module may include an enclosing structure or chamber arranged to provide a vacuum environment to support the plasma. Such a radiation system is typically termed a laser produced plasma (LPP) source. In an alternative system, which may also employ the use of a laser, radiation may be generated by a plasma formed by the use of an electrical discharge—a discharge produced plasma (DPP) source.
A proposed LPP radiation source generates a continuous stream of fuel droplets. As discussed above, a laser beam may be directed at these fuel droplets to generate a radiation generating plasma. However, all droplets in that stream may not be targeted by the laser beam, and thus some droplets may pass through and beyond a plasma formation location without being, for example, vaporised or the like by the laser beam. These fuel droplets need to be caught, and preferably caught in such a way that minimises or avoids splashing, which splashing could contaminate the radiation source (e.g. a collector forming part of the radiation source).
A proposed apparatus for catching fuel droplets involves the use of an open-ended tube, as for example shown in published international (PCT) patent application WO 2010/117858. The droplets are directed into this tube such that the droplets are incident at a grazing incidence angle on an internal surface of the tube, thus causing little or no back-splashing. However, the required or desired dimensions of the tube are linked to the speed of the droplets and also the accuracy of delivery of the droplets into the tube. If the droplets are faster, then the angle of incidence needs to be more grazing, and the tube needs to, in general, increase in length. Alternatively or additionally, if the accuracy of droplet delivery is not consistent, then the tube opening may need to be wider, in order to be able to catch each and every droplet and at the required grazing angle of incidence. One or both of these problems can lead to the need for a larger tube, in terms of the length and/or width of the tube. Space within a radiation source is at a premium, and it is not desirable in terms of space restrictions, design restrictions and cost to make the radiation source bigger simply to accommodate such a tube-based catching device.